This invention relates to a railway carrier apparatus for transporting semiconductor wafers which is contained in a wafer cassette.
In semiconductor producing plants, there are widely used railway carriers for transporting semiconductor wafers contained in wafer cassettes. These railway carriers generally utilize linear motors in order to enhance the cleanliness level of the clean rooms in the product plants.
FIG. 1 illustrates a typical example of the conventional railway carrier. Reference numeral 20 denotes a substantially flat guide rail which is supported by support bases 22. This guide rail 20 is provided on its upper face with a plurality of primary units 24 aligned at predetermined longitudinal distances. These primary units 24 constitute a linear motor 32 in cooperation with a secondary unit described later on, and produce magnetic fields for driving a carrier 26 along the guide rail 20. The carrier 26 has a carrier body 28 designed to straddle the guide rail 20, and also has a container 30 mounted on the carrier body 28 so as to contain a wafer cassette 50 in it. The secondary unit 34 of the linear motor 32 is attached to the inner face of the carrier body 28 so that the secondary unit 34 confronts the primary unit 24 on the guide rail 20. A plurality of wheels 36, 38. 40 and 42 are rotatably connected to the inner face of the carrier body 28, and are engaged with side faces of the guide rail 20 so that the carrier body 28 is movable along the guide rail 20. On the bottom wall 46 of the container 30, there is secured a flat bedplate 44 having engaging members 48 which are adapted to engage with the corners of a wafer cassette 50 so that the cassette 50 is stably placed on the bedplate 44.
As illustrated in FIG. 2, the wafer cassette 50 is a channel-shaped structure having an open top face, U-shaped opposite end faces, a closed bottom face and a curved inner face. A plurality of parallel grooves 51 are formed in the curved inner face of the cassette 50. In order to transport semiconductor wafers 52 by the carrier 26, the semiconductor wafers 52 are put into the cassette 50 in such a manner that the semiconductor wafers 52 are received in the grooves 51 of the cassette 50 and thus are parallel both to one another and to the U-shaped end faces. Then the cassette 50 is placed in the container 30 so that the semiconductor wafers 52 are aligned vertical and parallel to the longitudinal direction of the guide rail 20.
However, since the width of each cassette's groove is generally larger than the thickness of a semiconductor wafer 52, the wafers 52 are rocked due to inertia force or due to vibration transferred from the carrier 26, during the transference of the carrier 26, for example, upon the starting, the braking and the cornering of the carrier 26. As a result, the wafers 52 are rubbed against the inner wall of the grooves of the cassette 50, which causes such damage as abrasion of the semiconductor wafers 52.